1. Field of the Invention
This invention relates generally to electrochemical fuel cells and, more specifically, to an electrochemical fuel cell wherein the fluid flow design provides greater reactant contact with the fluid distribution layer in the outlet region than in the inlet region.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Solid polymer fuel cells typically employ a membrane electrode assembly (MEA) consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers, namely a cathode and an anode. The membrane, in addition to being an ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant streams from each other.
At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer to form a reaction product. The location of the electrocatalyst generally defines the electrochemically active area.
In electrochemical fuel cells, the MEA is typically interposed between two substantially fluid impermeable separator plates (anode and cathode plates). The plates typically act as current collectors and provide support to the MEA. The plates may have reactant passages formed therein and act as flow field plates providing access of the fuel and oxidant to the porous anode and cathode surfaces, respectively, and providing for the removal of product water formed during operation of the cells.
The conditions in an operating fuel cell vary significantly across the electrochemically active area of each electrode. For example, as the oxidant is consumed, water is produced, the total gas pressure normally decreases and the oxidant partial pressure decreases. This results in a greater current density in a region near the reactant inlet as compared to the reactant outlet. Performance of the cell may be limited by the high current density region, thereby resulting in a lower overall voltage than if the current density were uniformly distributed across the cell. High current density also results in increased local temperatures which tend to lead to greater material degradation. Higher temperatures may also result in a decrease in the relative humidity at the inlet, which can increase the likelihood of transfer leaks developing across the membrane and cause a localized loss of performance. This latter effect can be exacerbated if there is little or no humidification of the inlet gas streams. While the inlet portion of the cell is likely to be too dry, the outlet portion of the cell is likely to have too much water which can result in localized flooding, uneven performance and increased mass transport losses. Thus, the requirements and desired properties of the fuel cell electrode and flow field plate will vary across the fuel cell.
U.S. Pat. No. 5,840,438 which is incorporated herein by reference, discloses the fuel cell performance benefits of imparting different fluid transport properties in a fuel cell electrode substrate, in a biased manner, between a reactant inlet and outlet. U.S. Pat. Nos. 4,808,493 and 5,702,839 disclose varying the loading or composition of the electrocatalyst or other components, in a fuel cell electrode layer in a biased manner between a reactant inlet and outlet.
PCT Publication No. WO 00/31813 discloses an additional perforated plate interposed between a separator plate and an adjacent porous fluid distribution layer wherein the perforations in the plate vary in size. Japanese Publication No. 2001-043868 discloses increasing the cross-sectional area of the flow field path in the separator plates between the reactant inlet and outlet. Conversely, Japanese Publication No. 2001-006717 discloses decreasing the cross-sectional area of the flow field path in the separator plates between the reactant inlet and outlet. U.S. Pat. No. 6,048,633 discloses decreasing the effective cross-sectional area of reactant passages through the progressive convergence of flow field paths.
While a number of advantages have been made in this field, there remains a need for improved electrochemical fuel cells, particularly with regard to field flow distribution. The present invention fulfills this need and provides further related advantages.
In a typical fuel cell, reactants, either an oxidant or a fuel, flow from the inlet to the outlet through a plurality of reactant flow passages. The reactants diffuse from the reactant flow passages through a fluid distribution layer to a catalyst layer where the electrochemical reaction takes place. In typical fuel cells, the reactant flow passages are uniform in depth and width along their length. To improve the operating conditions of the cell, the reactant flow passages may be non-uniform in design. In particular the design can advantageously allow greater reactant access to the fluid distribution layer as the reactant flows from the inlet to the outlet.
In one embodiment, at least one reactant flow passage is narrower at the inlet than at the outlet. In order to maintain a substantially constant gas velocity, the reactant flow passage may become shallower as the passage widens to maintain a substantially constant cross-sectional area along the length of the passage. For example, the reactant flow passage may increase in width continuously from the inlet to the outlet. In another embodiment, the reactant flow passage may increase in width from the inlet to a point between the inlet and the outlet and thereafter maintain a substantially constant width to the outlet. Alternatively, the reactant flow passage can increase in width in a step-wise manner. In yet another embodiment, a reactant flow passage furcates to two or more shallower passages while maintaining a substantially constant total cross-sectional area.
The above embodiments can be used in a fuel cell comprising a fluid distribution layer comprising a porous, fluid permeable sheet material wherein the reactant flow passages comprise reactant flow passages on a surface of the separator plate facing the adjacent fluid distribution layer. Alternately, the fluid distribution layer may comprise a substantially fluid impermeable sheet material perforated in the active area. If a perforated, substantially fluid impermeable sheet material is used for the fluid distribution layer, the reactant flow passages may be formed in either a major surface of the separator plate facing the adjacent fluid distribution layer or in a major planar surface of the fluid distribution layer facing the adjacent separator plate.
Additional features may be present, such as, for example, coolant flow channels that mirror the reactant flow passages by being wider and deeper at the reactant inlet than at the reactant outlet or water transport features such as, for example, capillary channels or wicking fibres.
To accommodate the extra depth of the reactant flow passages at the inlet, the thickness of the plate may vary from the inlet to the outlet, and in particular, the plate may be thicker at the inlet than at the outlet. This may result in a substantially rectangular, wedge-shaped plate. In a fuel cell with two such plates, the inlet of the anode can be aligned with the outlet of the cathode.
In an alternate embodiment, a fuel cell comprises a conductive, substantially fluid impermeable masking foil superposed over at least one reactant flow passage in a region near the inlet and not extending the length of the fuel cell to the outlet.
These and other aspects of this invention will be evident upon review of the attached figures and following detailed description.